1. Field of the Invention
The present invention relates to an optical repeater and an optical signal regeneration method, and in particular to an optical repeater used for optical digital communication, and an optical signal regenerative repeater for regenerating and transmitting a digital signal light.
2. Description of the Related Art
For optical digital communication, when an optical communication signal (digital signal light) is transmitted along a transmission path made of an optical fiber, deterioration of the signal occurs due to effects produced by various factors, such as wavelength dispersion, nonlinear optical effects and timing jitter. Therefore, an optical light regenerative repeater is ordinarily employed to provide pulse regeneration for a received, deteriorated optical communication signal, and to obtain and transmit a signal light corresponding to the light communication signal before being deteriorated.
One such optical signal regenerative repeater is an optical 3R repeater having 3R functions, i.e., the reamplification, reshaping and retiming functions. Upon the reception of a deteriorated signal, the reamplification function reamplifies the intensity and the reshaping function reshapes the distorted waveform; and the retiming function, using the signal light, regenerates a clock signal that enables the transmission, at a correct time interval, of the reamplified and reshaped waveform.
Japanese published application H8-163047A, H. J. Thiele et al., Electron. Lett. 35, 230 (1999), and H. Kurita et al., Proc. of ECOC '99, PD3-6 (1999) disclose a general configuration for the optical 3R repeater as shown in FIG. 1. This optical 3R repeater comprises an optical branching device 101, a clock extraction device 102, and an optical gate 103. The optical branching device 101 divides a deteriorated optical communication pulse having a deteriorated wavelength of λ 1 into two pulses. One of the optical communication signal pulses obtained by the optical branching device 101 is supplied as control light to the optical gate 103, while the other optical communication signal pulse is transmitted to the clock extraction device 102. The clock extraction device 102 extracts the clock for the optical communication signal pulse received from the optical branching device 101, and generates an optical clock pulse, having a wavelength of λ 2, that is synchronized with the extracted clock. The optical clock pulse generated by the clock extraction device 102 is then transmitted to the optical gate 103.
In the above described optical 3R repeater, the optical gate 103 is opened or closed in accordance with the deteriorated optical communication signal pulse (control light), while the optical clock pulse received from the clock extraction device 102 is passed to the optical gate 103, and communication data are transferred to the optical clock pulse. In this manner, the discrimination and the regeneration of the deteriorated data are performed.
Recently, as a result of the rapid development of the Internet, a demand for the expansion of the optical communication capacity has been increased, and a fast optical gate operating at a transfer rate (bit rate) of 40 Gb/s or higher has been requested. Such a fast optical gate is available as a push-pull optical gate that uses light interference. By employing the configuration of this typical optical gate, the principles of the operation is described next.
K. Tajima, Japan J. Appl. Phys. 32, L1746 (1993) discloses an example of a push-pull optical gate in the form of a symmetrical Mach-Zehnder (SMZ) optical gate, as shown in FIG. 2. According to the SMZ optical gate, a control optical pulse (λ 1) is divided, by an optical branching device 201a, into first and second control optical pulses, and a controlled optical pulse (λ 2) is divided, by an optical branching device 201b, into first and second controlled optical pulses.
The first control optical pulse is transmitted directly to an optical coupler 204a and is coupled with the first controlled optical pulse. During this coupling process, the first control optical pulse is inserted, along the time axis, into a position preceding the first controlled optical pulse, and the second control optical pulse is delayed by an optical delay circuit 210 for a delay time Δ t, and is transmitted to an optical coupler 204b where it is coupled with the second controlled optical pulse. For this coupling process, the second control optical pulse is inserted, along the time axis, into a position following the second controlled optical pulse.
The first control optical pulse and the first controlled optical pulse arrive at a nonlinear optical phase shifter 211a in the this order, and the second controlled optical pulse and the second control optical pulse arrive at a nonlinear optical phase shifter 211b in this order. Furthermore, the timing at which the first and second controlled optical pulses arrive at the nonlinear optical phase shifters are the same, while the timing the first control optical pulse arrive at the nonlinear optical phase shifter is earlier than the timing of the controlled optical pulses, and the timing the second control optical pulse arrives at the nonlinear optical phase shifter is later than the timing the controlled optical pulses. Therefore, under these circumstances, the first and the second controlled optical pulses are sandwiched between the first and the second control optical pulses. At this time, the time interval between the first and the second control optical pulses is Δ t, and is employed as the gate width for the SMZ optical gate.
When the control optical pulses enter the nonlinear optical phase shifters 211a and 211b, a nonlinear phase shift occurs in the controlled optical pulses that arrive thereafter. Thus, in the nonlinear optical phase shifter 211a, the nonlinear phase shift occurs in the first controlled optical pulse due to the first control optical pulse, whereas in the nonlinear optical phase shifter 211b, a nonlinear phase shift does not occur because the second controlled optical pulse arrives before the second control optical pulse.
The first and the second controlled optical pulses, once they have passed through the nonlinear optical phase shifters 211a and 211b, arrive at the optical coupler 204c. Because of interference, the optical coupler 204c outputs an optical pulse only when there is a phase difference between the received controlled optical pulses. When there is no phase difference, the optical coupler 204c does not output an optical pulse because the controlled optical pulses cancel out each other. Since, in case there is the control optical pulse, there is a phase difference between the first and the second controlled optical pulses, the optical coupler 204c outputs an optical pulse (λ 2) obtained by synthesizing the controlled optical pulses. The optical pulse output by the optical coupler 204c passes through a wavelength band-pass filter (BPF) 212 and is output as gate passing light to the outside through the SMZ optical gate.
Generally, in the optical 3R repeater, the clock extraction device outputs controlled optical pulses at constant intervals. At the above described SMZ optical gate, as is shown in FIG. 3, of the controlled optical pulses that are sequentially input at constant intervals, the only controlled optical pulse sandwiched between the first control optical pulse (switch-ON) and the second control optical pulse (switch-OFF) is extracted.
K. Tajima et al., Appl. Phys. Lett. 67, 3709 (1995) discloses a polarization discriminating SMZ optical gate (PD-SMZ or UNI (Ultrafast Nonlinear Interferometer)) as shown in FIG. 4. At the polarization discriminating (PD-) SMZ optical gate, an optical polarization separator 213a divides a controlled optical pulse (λ 2) to obtain controlled optical pulses having first and second polarized components (TE mode and TM mode). The controlled optical pulse (TE; λ 2) is transmitted directly to an optical polarization coupler 214a, where it is coupled with the controlled optical pulse (TM; λ 2). The controlled optical pulse (TM; λ 2) is delayed by an optical delay circuit 210a for a delay time Δ t. The interval for the resultant controlled optical pulse (TE/TM; λ 2) is Δ t, and is employed as the gate width for the PD-SMZ optical gate.
The controlled optical pulse (TE/TM; λ 2), which is obtained by optical polarization coupler 214a, is transmitted to an optical coupler 214b and is coupled with a control optical pulse (λ 1), and thereafter, the thus obtained control optical pulse (λ 1) and the controlled optical pulse (TE/TM; λ 2) are received by a nonlinear phase shifter 211. The nonlinear phase shifter 211 receives the controlled optical pulse (TE), the control optical pulse and the controlled optical pulse (TM) in the named order. Therefore, at the nonlinear phase shifter 211, a nonlinear phase shift occurs only for the controlled optical pulse (TM), which arrives after the control optical pulse.
The controlled optical pulse (TE/TM), which passes through the nonlinear phase shifter 211, is again divided by an optical polarization separator 213b, and the controlled optical pulse (TM) is transmitted directly to an optical polarization coupler 214c, where it is coupled with the controlled optical pulse (TE). The controlled optical pulse (TE) is delayed by an optical delay circuit 210b for a delay time Δ t. Interference and synthesization is then performed for the resultant controlled optical pulse (TE/TM), and thereafter, an arbitrary linearly-polarized component is selected by a polarizer 215. Subsequently, the pulse of the selected linearly-polarized component passes through a wavelength band-pass filter (BPF) 212, and is output as gate passing light to the outside through the PD-SMZ optical gate.
At the PD-SMZ optical gate, the wavelength band-pass filter 212 may be positioned immediately after either the nonlinear phase shifter 211 or the optical coupler 214c. Further, in the above description, no problem is encountered when the TE mode and the TM mode are exchanged.
M. C. Farries et al., Appl. Phys. Lett. 55, 25 (1995) discloses another push-pull optical gate which is a nonlinear optical loop mirror shown in FIG. 5 (NOLM, SLALOM (Semiconductor Laser Amplifier in a Loop Mirror) or TOAD (Terahertz Optical Asymmetric Demultiplexer)). This nonlinear optical loop mirror has a loop structure, and a nonlinear optical phase shifter 211 is provided as a part of a fiber loop. A controlled optical pulse (λ 2), is provided into the fiber loop by an optical coupler 216 located opposite the nonlinear optical phase shifter 211 along the fiber loop, and at this time, the controlled optical pulse is divided into a first controlled optical pulse which is transmitted clockwise along the fiber loop, and a second controlled optical pulse which is transmitted counterclockwise.
The first controlled optical pulse arrives at the nonlinear optical phase shifter 211 directly, whereas before the second controlled optical pulse arrives at the nonlinear optical phase shifter 211, it is delayed by an optical delay circuit 210 for a delay time Δ t. Therefore, between the timings whereat the first and the second controlled optical pulses arrive at the nonlinear optical phase shifter 211 there is a time difference Δ t, and this difference is employed as a gate width.
An optical coupler 214 is provided along one transmission path connecting the optical coupler 216 and the nonlinear optical phase shifter 211, and a control optical pulse (λ 1) is introduced into the loop by the optical coupler 214. The control optical pulse is inserted, along the time axis, into a position after the first controlled light pulse, and the first controlled optical pulse, the control optical pulse and the second controlled optical pulse arrive at the nonlinear optical phase shifter 211 in the named order. Therefore, in the nonlinear phase shifter 211, a nonlinear phase shift occurs only for the second controlled optical pulse that arrives after the control optical pulse.
The second controlled optical pulse, which passes through the nonlinear phase shifter 211, arrives at the optical coupler 216, whereas the first controlled optical pulse, which also passes through the nonlinear phase shifter 211, is delayed by the optical delay circuit 210 for a delay time Δ t before it arrives at the optical coupler 216. Thus, the timings at which the first and the second controlled optical pulses arrive at the optical coupler 216 is the same, and the pulses interfere with each other. At this time, since there is a phase difference between the first and the second controlled optical pulses, an optical pulse (λ 2), obtained by synthesizing these controlled optical pulses, is extracted by the optical coupler 216 outside the loop. The thus extracted optical pulse then passes through the wavelength band-pass filter (BPF) 212, and is output as gate passing light to the outside the nonlinear optical loop mirror.
All of the fast optical gates described above are of a type whereby, due to the control optical pulse, a nonlinear phase shift occurs for the following controlled optical pulse, and a change in the interference state at the output port is employed to open or close the gate at high speed.
In addition to the above described optical 3R repeater, “IEEE Photonics Technology Letters”, Vol. 10, pp. 346-348, 1998 proposed by Y. Ueno et al. discloses a DISC (Delayed-Interference Signal-wavelength Converter), which is also included as an optical signal regenerative repeater. According to the DISC, based on input signal light pulses, a phase shift intermittently occurs along the time axis for continuous light having a wavelength differing from that of the input signal light pulses. Thereafter, the continuous light interferes with its replica by shifting the time, so that signal light pulses having a converted wavelength is obtained. Excluding the retiming function, this converter serves as an optical 2R repeater.
Furthermore, “IEEE Photonics Technology Letters”, Vol. 13, pp. 860-862, 2001 by J. Leuthold, et al., discloses a DISC that also serves as an optical 3R repeater by employing clock light pulses instead of continuous light, and by equaling to a bit period the time length that is to be shifted at the interference with the replicas.
Further, Japanese Published application 2002-229081A discloses an optical gate having a two-step structure, so that a sufficient intensity-noise suppression function can be performed for an optical signal, even when a phase shift obtained by the nonlinear phase shifter does not reach π.
In addition, Japanese Published application 2001-249371A discloses an optical gate having a two-step structure whereby the degree of freedom for wavelength conversion and the reliability for clock recovery are improved.
Depending on an increase of communication traffic, the transmission system can be changed from the WDM (Wavelength Division Multiplexing) system to the DWDM (Dense WDM) system, which divides a wavelength band into many more channels. In order to maximize a spectral efficiency in the DWDM system, the pulse time width of the optical communication signal pulse may be required to extend to about half that of the bit period, while it is maintained at the Fourier-transform limit.
The Japanese Published applications 2002-229081A and 2001-249371A mentioned above disclose a optical gate having a two-step configuration. However, there is no explanation for problems that arise when both a control light and a controlled light having a large pulse time width are employed in a DWDM system, such as problems associated with the shape along the time axis of the gate window, the timing jitter tolerance and the distortion of the regenerated output waveform.